First I would say remember this is your fathers business computer and he will be using it mainly for AutoCAD. So even though you do like Macs the Mac brand would be superfluous as he will be spending most if not all his time in Windows.
Then I would say build the AutoCAD PC yourself. It would be much cheaper than a Dell, more upgradeable and you can use high grade components, such as the motherboard and PSU. Dell just uses whatever the lowest bidder offers that meets requirements.
For about then $1300 you can build your father an very high quality Core 2 Quad workstation. This includes 24" widescreen, 4GB RAM, 1TB RAID 5 array and Windows Vista Business x64 (which can get the free upgrade to Win 7 when it is released). For around $1450 you can go Core i7, more expandable motherboard and 6GB DDR3.
Are you nervous about building the computer. It is quite easy, screw some stuff to the case, plug everything in, look at your manual to set up RAID 5 in the BIOS, then install Windows, install the software and drivers. I already did the most difficult part for you by finding the parts.
Parts:
CPU;
Core 2 Quad Q9550 2.83Ghz $220
RAM;
4GB DDR2 800 Corsair XMS2 $55
Motherboard;
Gigabyte GA-EP45-UD3P $135
PSU;
Corsair 650W 80+ certified $100
Case;
Cooler Master Centurion $45
Hard Drive;
Hitachi 500GB (get three for 1TB RAID 5) $165 for all 3
Monitor;
Samsung 24" good reviews $310
Video Card;
XFX Radeon 4650 $55
DVD;
Samsung DVD DL $26
OS;
Win Vista Business x64 (includes Win 7 free upgrade voucher) $150
Shipping about $50
Total $1261
Core i7 System:
CPU
Core i7 920 2.66Ghz $280
Motherboard;
MSI X58 Pro-E $190
RAM:
6GB (3x2GB) DDR3 1600 OCZ Gold $100
Total $1421
Notes:
- Don't bother saving a few bucks with a Dual Core CPU or less than RAID 5. After the cost of everything else the price decrease of the downgrades is negligible but the performance decrease, hence lifespan, is massive.
- Budget for other upgrades if needed (Printer, Network, Back up drives). Go to the websites for your fathers peripherals and make sure they have Vista x64 drivers and then Google search to see if those peripherals work in Windows 7 x64 RC.
- I included RAID 5 due to the faster read/write and if a drive fails the computer does not need to have Windows, software and backups re-installed. Instead you just replace the failed drive.
- I included a 24" screen since that 1920x1200 can be quite beneficial in his work flow compared to a 20" 1680x1050.
- I included Vista Business for Remote Desktop Client support
- Software
* Budget for software upgrades as well like Office and AutoCAD 2010
* If your father needs MS Exchange support he may want MS Office 2007 Small Business Edition if he does not need MS Exchange and Outlook then the free Open Office may work fine.
* Anti-Virus: Get Avast Antivirus free edition it works great, it is free, and uses very little resources. I install it on my own, friends, family and customers computers.
* Other software such as Quickbooks
* Does your father use the 3D visualization features in AutoCAD he may benefit in an NVidia Quadro graphics card since AutoCAD 2010 includes CUDA support for 3D work.